But you'd probably consider someone 10 months older than you, or 15 months older than you, to be "a year older." So if your birthday is near the beginning or end of the year, those people could be born in 1998 or 2000.

And if we're talking about numerical age, which abruptly jumps in 1-year increments on each person's birthday, then which people are "a year older" than you changes every day.
Example:
Alice was born in Nov 1998, Bob was born March 1999, Chris was born Sep 1999, Dana was born April 2000, you were born June 2000.
In Jan 2019, Alice is 20 years old, Bob and Chris are 19 years old, Dana and you are 18 years old. That means that Bob and Chris are one year older than you.
In May 2019, Alice and Bob are 20, Chris and Dana are 19, you are 18. That means that Chris and Dana are one year older than you.
In July 2019, Alice and Bob are 20, Chris, Dana, and you are 19. That means that Alice and Bob are one year older than you.